where to find all those #ifdef's and #defines

[David Whitney- RCD]

No seemed to know before, but I'm sure somone's _gotta_ know somewhere:

I've got a lot of programs that run on different systems and I want to find
out where each is running (among other things).  I've seen a lot of
cpp commands to determine some things (e.g.

#ifdef CRAY
...do such and such
#endif

Where is "CRAY" defined?  Or "unix" or "sgi", etc.?

Does anyone know who, what, or where to find these definitions for each
machine?  What I'd like (ideally) is a list of the variables that
are #define'ed at any one time.  Thanks,

-Dave

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